Reaver

"Take hold of my hand, for you are no longer alone. Walk with me in hell."

Magic is a gift, whether given or asked for. You did not ask for it, you took it. Carved into your very flesh is the mark of your thievery, the visual lock of the supernatural being imprisoned within you. It lies caged within you and you must learn to tighten its leash. This being could be a hapless fiend, a mindless undead, or a misbegotten celestial. Whatever form it takes, it must learn its place as the source of your power. If it doesn't, it very well might kill you. 

Level 3: Relentless Onslaught

You gain proficiency in Martial Weapons and Medium Armor. In addition you gain the Dual Wielder feat.

Reaver Spells

Your imprisoned entity grants additional spells. When you reach a Warlock level specified in the Reaver Spells table, you thereafter always have the listed spells prepared.

Level 3 - Blur, Flame Blade, Hellish Rebuke

Level 5 - Blink, Vampiric Touch

Level 7 - Fire Shield, Dominate Beast

Level 9 - Immolation, Far Step

Level 3: Set Fire to the Pain

You gain resistance to fire damage. If a source would deal damage to you, you can make a Constitution saving throw equal to your spell DC. If you succeed, the damage type changes to fire.

Level 3: Focused Hatred

You level your locus of slaughter to a single target. Once per long rest, you choose a target that you can see and mark them as your prey. You have advantage on melee and spell attacks against the target. If the target would die by anyone else other than you, you may choose one other target for this feature. You can only use this feature a number of times equal to your proficiency bonus.

Level 6: Undying Embers

The hellfire within burns eternally, none can escape its conflagration. As a bonus action on your turn you can spend a hit die to regain the use of an expended spell slot.

Level 10: Dark Metamorphosis

Sometimes you have to let the demon within out. Your size doubles, along with any equipment you are wearing. You sprout wings of pure hellfire and gain a fly speed of 60 feet. You gain half your current hit point total as temporary hit points. This form lasts for 1 minute, during that time you become a fiend in addition to your species.

Level 14: Chaos Theory

Nothing can remain in your wake, nothing. Once per long rest, as an action, you can use this feature. When you do, at the beginning of each of your turns after you activate this feature you burst in a wave of hellfire. Every creature, including you, within 30 feet must take 4d6 fire damage. As an action, you can deactivate this feature.